Various types of metals are used for components which may be exposed to a high-temperature environment, such as aircraft engine parts. A variety of approaches have been used to raise the operating temperature at which the metal components can be used. For example, one approach involves the use of protective coatings on various surfaces of the component, e.g., a gas turbine engine airfoil. The coatings are usually ceramic-based, and are sometimes referred to as thermal barrier coatings or “TBC's”.
The TBC's are typically used in conjunction with internal cooling channels within the airfoil, through which cool air is forced during engine operation. As an example, a pattern of cooling holes may extend from a relatively cool surface of an airfoil to a “hot” surface which is exposed to gas flow at combustion temperatures of at least about 1000° C. The technique is sometimes referred to as “discrete hole film cooling”.
In some instances, cooling structures must be located relatively close to a “hot” surface—even when such a surface is covered by a protective coating. For example, a coolant such as air is often passed through the inner surfaces of a turbine combustor liner, as well as being directed over an outside surface of the liner (i.e., “backside” cooling). This type of cooling mechanism is generally described in U.S. Pat. No. 5,822,853 (Ritter et al). The combustor liner usually has a thickness of about {fraction (1/10)} inch (2.5 mm) to about {fraction (3/16)} inch (4.7 mm).
Much work is being undertaken to design suitable patterns of cooling channels for thin-walled structures. For example, the Ritter patent describes the formation of cooling channels in cylindrical structures, such as the turbine combustors. In the Ritter technique, a double-wall assembly is constructed. The assembly includes an inner wall, channel-forming means, a sacrificial channel filler, and an outer wall. The channel-forming means lies between the two walls, and is filled with the channel filler. The assembly is hot-pressed to bond each wall to the other. Subsequent removal of the filler results in the desired cooling channel.
U.S. Pat. No. 5,075,966, issued to Mantkowski, also describes a method for making cooling channels and other hollow structures. The surface of a substrate is selectively patterned, and the pattern is then filled with a slurry. When solvent is removed, the slurry is transformed into a solid filler. A close-out layer is then deposited over the patterned surface and filler. Subsequent removal of the filler results in the formation of the desired hollow structure. The Mantkowski process can be employed to form hollow cooling channels within turbine engine components, e.g., blades and vanes.
There are certainly advantages associated with the technology embodied in the Mantkowski and Ritter patents. However, those processes may exhibit some disadvantages as well, for some applications. For example, the processes usually require the formation of grooves or channels in a metal surface. These features typically need to be patterned and then formed by some sort of casting or machining process. Casting and machining can be time-consuming. Moreover, these techniques are not always suitable for small-diameter structures, or for structures with complicated shapes. The limitations of these processes become more apparent when they are considered for use in advanced applications, e.g., channels in thin-walled, superalloy structures.
Thus, new methods for the formation of hollow structures in coated metal substrates would be of considerable interest in the art. The methods should obviate the use of casting or machining processes. They should also be capable of forming patterns of the structures, e.g., complicated patterns of cooling channels. The methods should also be compatible with processes used to apply the coatings to the substrates. Moreover, it would be advantageous if the new methods allowed one to easily change the shape of the hollow structures, or the composition of the material which forms the hollow region itself. This flexibility would be very useful for enhancing heat transfer characteristics, in the case of cooling channels for gas turbine applications.